An optical-axis deflection type laser interferometer includes a reference ball serving as a measurement reference, a retroreflecting means disposed at an object to be measured, a laser interferometer length measuring apparatus outputting a measurement value between the apparatus and the retroreflecting means, and a rotational mechanism that rotates a beam projected from the laser interferometer length measuring apparatus around the reference ball. The laser interferometer additionally includes a displacement gauge that measures an error caused by a relative motion between the reference ball and the laser interferometer length measuring apparatus, and a rectilinear movement mechanism that displaces these in a direction of an optical axis of a measurement beam with respect to the reference ball while maintaining a relative positional relationship between the laser interferometer length measuring apparatus and the displacement gauge. With this structure, calibrating the displacement gauge without preparing a special calibrating device becomes possible, thereby securing traceability thereby securing traceability.